Women on higher doses of progestin had greater endurance and longer lasting energy, the study found.

The high-dose pill also stimulated the burning of fat, rather than carbohydrate, as an energy source.

"This study provides the first evidence that synthetic progestins in oral contraceptive pill formulations can have a significant affect on maximal aerobic power," the researchers write in the study, which is published online ahead of in print.

Is the pill a performance enhancing drug?

Norman says although the study used non-athletes, it raises the prospect of elite athletes improving their performance with a substance accepted by international sports drug authorities.

"It raises the fact that yet again for drug testing we need to decide what's legitimate and what's not legitimate and where do we draw the line," he says.

"What this does for the first time is tell us that there may be 'legal ways' of changing the pill that athletes are on, that may improve or decrease their performance."

He says the pill is accepted as a legitimate way of providing elite athletes with the oestrogen they may be lacking because of their heavy training and low body weight.

The pill not only regulates their periods but also protects them against the bone thinning disease osteoporosis.

He acknowledges the study may have thrown a spanner in the works.

"It does open up a whole new area as to what is legitimate treatment for women's physiological disorders," he says.

Norman says there's a "remote" risk that sports drug authorities could ban the pill, although this would be unlikely.

"There may be a pill that turns up or is specifically designed to improve performance and the authorities might target that particular pill. But I very much doubt that they'll ever be able to touch the generic pills that are on the market," he says.

Only for the elite

Norman stresses that the exercise changes observed in the study aren't likely to mean much for your average three-times-a-week jogger or someone who occasionally cycles to work.

"The exercise changes that we got, although significant, are relatively minor," he says.

A woman's exercise performance is linked to her hormone levels whether she's on the pill or not, Norman says.

He says research leading to the current study found women exercise better in the second half of their menstrual cycle, after ovulation, than in the first half.

There is also evidence that the benefits extend to early pregnancy, when a woman's hormone levels shoot up to at least as high as the second half of a non-pregnant cycle.

In fact, the hormone changes produced in the study were equivalent to early pregnancy, he says.

More work needed

Dr Helena Teede, director of research at Australia's Jean Hailes Foundation, says it is unclear which component of the pill used in the study is responsible for the benefits.

She says while the research is interesting, women shouldn't conclude from one small, preliminary study that the pill will make them better at sport or lose fat.

"Rather ... it may help them be more efficient and effective in exercise but this needs more work," she says.